In general, carburetors are useful for small two-stroke internal combustion engines commonly used in hand held power tools such as chain saws, weed trimmers, leaf blowers, and the like. Carburetors control engine power by metering and mixing liquid fuel with air to adjust a fuel-to-air ratio and quantity according to varying engine requirements during startup, idle, steady-state operation, and for changes in load and altitude.
In particular, a scavenging carburetor is known to have a fuel-and-air mixture passage and a separate scavenging air passage that both communicate at one end of the carburetor with a clean air source at atmospheric pressure, such as air filter. At an opposite end of the carburetor, the fuel-and-air mixture passage and the separate scavenging air passage communicate with an engine crankcase and an engine combustion chamber, respectively. A butterfly throttle valve is disposed within the fuel-and-air mixture passage and a butterfly air control valve is disposed within the scavenging air passage.
To ensure stability of engine operation at idle and over a relatively low speed range, and ensure prompt acceleration from low to high engine speed, it is known to prevent the supply of scavenging air over a small angle range of increased opening of the throttle valve from its idle position. This delay in the opening of the scavenging valve enables a relatively rich fuel-and-air mixture to be supplied to the engine for stable combustion at idle and initial acceleration from idle. In other words, upon initial acceleration of the engine, the throttle valve opens to provide an enriched fuel-and-air mixture to the engine and, toward the end of initial acceleration, or shortly thereafter, the scavenging valve starts to open to permit clean air to flow by the scavenging valve for scavenging exhaust gases from the combustion chamber. This valve opening sequence thereby provides a relatively leaner mixture of fuel-and-air in the combustion chamber of the running engine after initial acceleration from the operating condition.
During steady state running of the engine at relatively higher speeds, the scavenging valve is open to supply a relatively large amount of scavenging air to the combustion chamber. This prevents the fuel mixture from blowing through the exhaust port of the engine, thereby reducing hydrocarbon exhaust emissions from the engine.
To meet different engine idling and high speed valve opening requirements, it is known to provide a lost-motion mechanism between a butterfly throttle valve and a butterfly scavenging valve to achieve delayed opening of the scavenging valve and synchronization between the throttle valve and scavenging valve. It is thereby possible to prevent the supply of the scavenging air until the opening angle of the throttle valve increases to a certain extent in a small opening angle range from idling by opening the scavenging valve only after the throttle valve has opened to a certain extent by using the lost motion mechanism.
Using conventional butterfly valve constructions and configurations, it is possible to prevent flow of scavenging air over a low speed range of the engine to achieve stable idle and low speed operation and stable initial acceleration, but there remains a problem during high speed operation. Specifically, over a high speed range of the engine, use of the butterfly-type scavenging valve does not enable a relatively large rate of change of the opening area of the butterfly-type scavenging valve compared to a relatively small rate of change of the opening area of the butterfly throttle valve. In short, conventional carburetor valve arrangements do not allow a favorable relationship between the opening areas of the throttle valve and scavenging valve to be achieved so as to meet different needs of a scavenged internal combustion engine operating at different speed and load conditions.